To operate effectively, navigation systems that rely on Global Position System (GPS) signals require signal faults within the system to be minimized. In particular, the probability of transmitting Misleading Signal in space Information (MSI) must be kept low for all signals transmitted by GPS space vehicles. MSI is defined as a User Range Error (URE) significantly greater than the broadcast User Range Accuracy (URA) bound that persists for a specified time period without the space vehicle switching to Non-Standard Code (NSC).
FIG. 1 shows a fault tree of possible GPS signal faults that can result in MSI, based on: Dr. Gary McGraw et al., Boeing Air Traffic Management, “Safety of Life Considerations for GPS Modernization Architectures,” ION GPS 2001, 11-14 Sep. 2001, Salt Lake City, Utah. These signal faults are grouped in the following categories: bad data upload; space vehicle data errors; space vehicle clock faults; signal distortions; and environmental errors. Bad data upload errors include errors in ephemeris and clock data received from ground stations. Space vehicle data errors include data processor faults and orbit propagation errors that occur within the space vehicle. Space vehicle clock faults include clock signal jumps and drifts within the space vehicle. Signal distortions include chip “ringing,” chip edge jitter, chip timing errors, excessive noise phase, and low power levels. Environmental errors include a variety of errors caused by atmospheric conditions and well as multipath and other types of interference.
A GPS On-Board Integrity Monitoring System (IMS) can be employed to assure that the probability of transmitting misleading signal in space information (MSI) shall be low for all signals that the GPS space vehicle transmits. One approach is to provide an IMS that uses a full GPS receiver to monitor the transmitted signals for signal distortions and clock faults. The data that the space vehicle Mission Data Unit (MDU) intended to transmit is compared with the data that the IMS receives from the actual transmitted signal. Some form of “reasonableness” check of the data uploaded from the ground stations and of the ephemeris data propagated by the MDU can be performed.
The full GPS receiver used for this purpose would be based on existing receiver designs, which have RF front ends that are unnecessarily complicated for an on-orbit IMS. In order to monitor GPS military signals, such an IMS might employ an embedded COMSEC device, which greatly increases the complexity of the device and requires COMSEC certification. Moreover, in order to determine the integrity of clock signals, such an IMS could use a high-accuracy clock source to check the clock signals of the on-board time keeping system. This independent timing reference would add complexity, power, and weight to the receiver. Further, the “reasonableness” check of the data uploaded from the ground stations and of the ephemeris data propagated by the MDU may not provide strong integrity assurance, and therefore a higher degree of integrity assurance on the MDU and its software may be required.
Accordingly, there remains a need for an integrity monitoring system with minimal weight, cost, power consumption, and complexity that can readily be incorporated into a transmitter system to be monitored, such as a GPS space vehicle. Moreover, other types of signal transmission systems could benefit from an enhanced capability to monitor data upload errors, system data errors, clock faults, or signal distortion.